This application claims the priority of Japanese Patent Application No. 2000-339955 filed on Nov. 8, 2000, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an imaging zoom lens for a camera employing an imaging device such as a CCD or image pickup tube, a silver halide film, or the like, or a projecting zoom lens for a projection type TV. In particular, the present invention relates to a projecting zoom lens used in a projection type display apparatus employing a liquid crystal, and a projection type display apparatus using the same.
2. Description of the Prior Art
Known as an example of conventional zoom lenses is one disclosed in Japanese Unexamined Patent Publication 05-297276 or the like comprising a negative first lens group, fixed upon changing power, having a focusing function; a positive second lens group having a power-changing function; a negative third lens group for correcting the image surface movement generated by the change in power; and a fixed positive fourth lens group.
However, most of such zoom lenses have been designed for use in imaging devices having a small size such as CCD. For using these lenses as a projection lens in a projection type display apparatus, it is necessary to increase the reduction-side size of the lens on which the image to be projected is focused, whereby the size of the lens itself becomes considerably large. When use in a projection lens is considered, the conventional technique is often short of distortion correction.
When illumination systems are also taken into consideration, it is desirable that a substantially telecentric optical system be provided on the reduction side of a projection lens in an apparatus using a liquid crystal. However, most of the conventional techniques have not given such consideration. Further, those provided with a back focus allowing a color-decomposing or color-combining optical system to be inserted between a lens system and an imaging surface are few.
For overcoming such a problem, a zoom lens disclosed in Japanese Unexamined Patent Publication No. 10-268193 has been known, which comprises a negative first lens group, fixed upon changing power, having a focusing function; a positive second lens group, a positive third lens group, and a negative fourth lens group which are movable with a mutual relationship for continuously changing power and correcting the image surface movement generated upon the continuous change in power; and a positive fifth lens group fixed upon changing power; and satisfies a predetermined conditional expression.
In recent years, however, there has been a demand for projecting images from a position closer to a large-size screen by using a projection lens having a wider angle. Though the zoom lens disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 10-268193 is supposed to be one achieving a compactness of the lens system for the reduction-side size, an appropriate amount of back focus, and a substantial telecentricity on the reduction side, its angle of view is 23 to 25 degrees, whereby it may not fully satisfy such a demand. Also, there has recently been a strong desire for reducing the size of projection type display apparatus, whereby the projection lens has further been required to reduce its size.
In view of such circumstances, it is an object of the present invention to provide a zoom lens having a compact configuration with respect to its reduction-side size and a wider angle of view than that conventionally available, while various aberrations are favorably corrected, back focus is long, and light beams within a tangential plane on the reduction side are substantially made uniform with respect to the optical axis. It is another object of the present invention to provide a projection type display apparatus using the above-mentioned zoom lens.
The present invention provides a zoom lens comprising successively from an enlargement side:
a first lens group, fixed upon changing power, having a negative refracting power for focusing;
a second lens group having a positive refracting power, a third lens group having a positive refracting power, and a fourth lens group having a negative refracting power which are movable with a mutual relationship for continuously changing power and correcting an image surface movement generated by the continuous change in power; and
a fifth lens group, fixed upon changing power, having a positive refracting power;
the zoom lens satisfying the following conditions (1) to (4):
xe2x88x921.5 less than F1/F less than xe2x88x920.7xe2x80x83xe2x80x83(1) 
0.5 less than F2/F less than 1.5xe2x80x83xe2x80x83(2) 
5.0 less than F3/F less than 15.0xe2x80x83xe2x80x83(3) 
0.8 less than F5/F less than 1.5xe2x80x83xe2x80x83(4) 
where
F is the focal length of the whole lens system at a wide-angle end;
F1 is the focal length of the first lens group;
F2 is the focal length of the second lens group;
F3 is the focal length of the third lens group; and
F5 is the focal length of the fifth lens group.
Preferably, the distance between the second and third lens groups is narrowed toward a telephoto end, whereas the second and third lens groups satisfy the following conditional expressions (5) and (6):
0.1 less than D2/F less than 0.5xe2x80x83xe2x80x83(5) 
0.03 less than xcex4D2/(Fxc3x97Ft)xc2xd less than 0.1xe2x80x83xe2x80x83(6) 
where
D2 is the lens distance between the second and third lens groups at the wide-angle end;
xcex4D2 is the absolute value of amount of change in the lens distance between the second and third lens groups in the area ranging from the wide-angle end to the telephoto end; and
Ft is the focal length of the whole lens system at the telephoto end.
Preferably, the third lens group comprises two lenses having positive and negative refracting powers, respectively, which are separate from each other or cemented to each other, and satisfies the following conditional expression (7):
xcexdN less than 35xe2x80x83xe2x80x83(7) 
where
xcexdN is the Abbe number of the lens having a negative refracting power in the third lens group.
The present invention provides a projection type display apparatus comprising a light source; a light valve; and a projection lens for projecting onto a screen an optical image formed by light modulated by the light valve; wherein the projection lens is the above-mentioned zoom lens.